Regarding the wafers used for wafer inspection, the resist is removed from the wafer edge in several processing steps by means of EBR (Edge Bead Removal). In this way, an edge bead removal line is generated. As a result, the wafer surface is stepped, which, in the acquired wafer image, can be seen as an ellipse or ideally as a concentric circle.
It is often difficult to find defects or anomalies on the wafer. Specifically, with respect to the edge area of the wafer, the EBRs of the edge area generally do not represent any defect for the user, but at the same time anomalies in this edge area should be detected. However, a problem in detecting such defects or anomalies in the edge area consists in distinguishing clearly between EBR and defect. Correspondingly, it is also often difficult to distinguish clearly between defects and desired structures on the wafer on the rest of the surface of the wafer.
Patent application DE 10 2005 014 594 discloses a method of detecting incomplete edge bead removal from a disk-like object, for example a wafer. For this purpose, first a peripheral area of a disk-like reference object is imaged. Marks are then defined in the peripheral area of the reference object. Finally, images of peripheral areas of a plurality of disk-like objects of the same batch are acquired. The inspection of the disk-like objects is limited to the locations of the marks defined on the reference object.
Patent application DE 10 2006 042 956 discloses a method of visualizing measured values from acquired images of disk-like objects. First an acquired image of at least one disk-like object is acquired, and a plurality of measured values is generated. A color value is assigned to each measured value. Finally, a resulting image is generated, wherein a color value selected from a predetermined range is assigned to an area that yielded a measured value on the disk-like substrate.
U.S. Pat. No. 5,859,698 discloses a method and an apparatus for macro-defect detection, wherein scattered light is used for image acquisition. The macro-defects may be located on a wafer, a partly processed wafer or on a liquid crystal display element. These defects are detected using scattered light. By means of an automated image processing technique, a reference image and a sample image are generated from the scattered light data. Then a difference image is generated by comparing the reference image and the sample image. The difference image is evaluated using at least one automated image processing technique, such as thresholding, morphological transformation and blob analysis, to identify macro-defects. However, only a transformation within the x-y coordinate system is performed instead of a polar coordinate transformation.
Japanese patent no. H04-123454 discloses a method for analyzing spurious particles or defects on a wafer. An interface is provided for converting the file and for the coordinate transformation of the spurious particle coordinate data to allow quick detection of spurious particles on the wafer. For this purpose, a wafer is inserted into a dust detector to detect the spurious particles. The x-y coordinates of the detected particles on the wafer are then stored in a storage medium. Then the wafer is removed from the detector and deposited on an automatic table of an inspection device. At the same time, the storage medium is removed from the detector and inserted into a computer connected to the inspection device to perform a coordinate transformation by converting the file containing the coordinate data. As soon as a spurious particle is identified, a control element is actuated to move the automatic x-y table such that the particle may be moved into the field of view and an image may be generated on the display. Again, only a transformation within the x-y coordinate system is performed instead of a polar coordinate transformation.